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January 10, 2005

Star Dubbed ‘King of Spin’ by Research Team

by Sam Savage

Boulder -- New ultraviolet observations indicate a Milky Way star is spinning nearly 200 times faster than Earth's sun, the probable result of a merger between two sun-like stars whose binary orbit recently collapsed, according to a University of Colorado at Boulder astronomer.

The yellow giant, known as FK Comae Berenices, or FK Com, is 10 times larger than the sun and is emitting spectacular amounts of X-rays, ultraviolet light and radio waves as it rotates furiously, said Senior Research Associate Thomas Ayres of CU-Boulder's Center for Astrophysics and Space Astronomy.

Dubbed the "King of Spin" by the research team, FK Com is the namesake of a rare class of fast-rotating yellow giants noted for high levels of coronal magnetic activity, said Ayres. "FK Com objects are oddballs because most giant stars rotate very slowly. That's why many theorists now believe binary mergers are the best way to explain the existence of these rare, ultra-fast rotators," he said.

Ayres presented a paper on the subject at the 205th meeting of the American Astronomical Society held Jan. 9 to Jan. 13 in San Diego. Co-authors on the paper include Alex Brown and Graham Harper of CU-Boulder's CASA, Heidi Korhonen of Germany's Potsdam University and Seth Redfield of the University of Texas, Austin.

"The negative feedback of magnetic activity on the orbit of stars in close binary systems must produce a number of these objects throughout our galaxy," Ayres said. "It's fortunate to have one that is so near to its birth, actually its rebirth, and which is close enough to Earth to be observed in detail."

FK Com is located 800 light-years from Earth, relatively close by galactic standards, he said. One light year is about 6 trillion miles.

The team observed FK Com with the Far Ultraviolet Spectroscopic Explorer satellite, or FUSE, launched in 1999 to probe the far UV radiation portion of the light spectrum invisible to the Hubble Space Telescope. The researchers used FUSE to observe the star for four hours on Feb. 12, 2004.

The rotation of FK Com recorded in UV light is roughly 200 miles per second, about twice that measured previously in the visible part of the spectrum, he said.

FK Com is covered with dark regions that are similar to sunspots but much larger and more pervasive, Ayres said. "This object seems to be flaring practically all of the time and often doubles or triples its vast X-ray output, which is already 100,000 times that of the sun, during these episodes."

The speedy rotation seen in the UV spectrum likely is caused by the star's slingshot prominences, which are massive, hot tubes of gas similar to magnetic loops that shoot out from the surface of the sun into its corona, he said. Up to several million miles long, the prominences are whipping around the star at much higher speeds than the surface rotation, much like the outer edge of a phonograph record moves faster than the inner part.

The research team believes the coronal wind near such prominences is carrying away large amounts of angular momentum, or spin, from the star. This eventually will "brake" the star's fast rotation, much like a twirling figure skater extends her arms in order to slow down, Ayres said.

FK Com is an "extreme case" of sun-like magnetic activity, he said. "By examining the extremes closely, we can gain insight into the inner workings of these violent cosmic phenomena, which studies of our own, very inactive sun could not reveal."

The international FUSE mission involves science team members from the United States, France and Canada. Four FUSE telescopes collect and funnel UV light into a $9 million spectrograph, which was designed and built at CU-Boulder and breaks down the light like sunbeams passing through a prism.

The FUSE mission is managed and operated for NASA by Johns Hopkins University in Baltimore, Md. FUSE observations and research on FK Com was supported by NASA through the FUSE Guest Investigator program.